Inorganic oxide coatings, such as those based on silica, are commonly applied to a substrate. They can be used as single layer or as part of a multi-layer coating (or coating stack) to add a specific functionality to the substrate. Examples of such functional coatings or functional coating stacks are sodium-blocking coatings, oxygen-barrier coatings, hard coats, and optical coatings, e.g. anti-reflective coatings. The ability of these inorganic oxide coatings to maintain their functional performance during use is often crucial to the viability of technology relying on such coatings. In particular, the economic feasibility of solar panels is sensitive to the ability of the solar panels to maintain high functional performance (i.e. generate electrical or thermal energy from sun light) over an extended period. Significant improvements in functional performance of solar panels have been achieved through the application of anti-reflective (AR) coatings on the cover glass. Typical single layer AR coatings are thin porous silica layers, and have for example been described in EP0597490, U.S. Pat. No. 4,830,879, U.S. Pat. No. 5,858,462, EP1181256, W02007/093339, WO2008/028640, EP1674891 and WO2009/030703.
These types of coatings, however, may be sensitive towards hydrolysis, making them less suitable for long-term outdoor application. Prolonged exposure to outdoor conditions typically leads to the formation of coating defects, and hence to a reduction in functionality and aesthetics of the coated substrate. Hydrolytic degeneration of the coating can be further accelerated by compounds that migrate from the substrate into the silica-based AR coating. In case of float glass, for example, sodium and calcium ions migrate into the coating, especially during thermal curing. These elements are known to accelerate hydrolytic degeneration of the silica coating.
One way to improve the hydrolytic stability of such porous silica coatings is the application of a topcoat. For example, US2008/0193635 discloses a process wherein a layer of amorphous diamond-like carbon is deposited on an anti-reflective coating to maintain efficient conversion of radiation in solar cells or panels. However, this technology requires the coating to be formed by anodization and consequently the process suffers from high cost and difficulties in scaling-up to the size required for meeting growing demand. Alternatively, more simple hydrophobic top coats can be applied to an AR coating, but such additional coating step still leads to increased production costs. In addition, this type of coating may deteriorate the aspired functionality. Furthermore, these coatings typically contain organic components such as fluoroalkyl compounds; the UV sensitivity of such compounds affecting durability of the coating stack.
A second approach to improve the hydrolytic stability is the application of a barrier film between the silica coating and the substrate, to reduce migration of alkali components; like a dense silica or mixed oxide layer. Such layer, however, also needs to be applied and cured in a separate coating step; leading to an increase in production costs. Additionally, such coating may be incompatible with the aspired functionality.
A further way to improve the hydrolytic stability of an inorganic oxide like silica is the addition of other elements, which replace part of the Si (or other) atoms in the network. It is known that a mixed oxide of silica and alumina shows improved resistance to hydrolysis; see for example R. K. Iler, The Chemistry of Silica, Wiley New York (1979). A disadvantage of this method is that mechanical properties may be negatively affected. Furthermore, addition of other inorganic oxide precursors like aluminum salts may reduce the stability of the coating composition prior to application; especially stability of a liquid coating composition comprising the inorganic oxide precursors for use in a so-called sol-gel process.
A sol-gel process, also known as chemical solution deposition, is a wet chemical technique that is typically used for making a (porous) inorganic oxide layer starting from a chemical compound in solution or colloid (or sol) form, which acts as precursor for forming an integrated network (or gel) of either discrete particles or network polymers. In such process, the sol gradually evolves to a gel-like diphasic system containing both a liquid and solid phase. Removing remaining liquid (drying) is generally accompanied by shrinkage and densification, and affects final microstructure and porosity. Afterwards, a thermal treatment at elevated temperature is often needed to enhance further condensation reactions (curing) and secure mechanical and structural stability. Typical inorganic oxide precursors are metal alkoxides and metal salts, which undergo various forms of hydrolysis and condensation reactions. Metal is understood to include silicium within the context of this description. To increase and control porosity and pore size, pore forming agents may be added (in addition to solvent). In processes for making an anti-reflective layer on a substrate generally coating compositions are applied that comprise only low amounts of components that will form the final solid layer, e.g. a solids content of up to about 10 mass %.
There is thus a need in industry for a coating composition that enables making an inorganic oxide coating on a substrate, like an anti-reflective layer on a transparent substrate, which coating shows improved hydrolytic stability.
It is therefore an objective of the present invention to provide such an improved coating composition.
The solution to the above problem is achieved by providing the composition and process as described herein below and as characterized in the claims.
Accordingly, the present invention provides a coating composition comprising:    an inorganic oxide precursor AMOx based on at least one inorganic element A selected from the group consisting of aluminum, silicium, titanium, zirconium, niobium, indium, tin, antimony, tantalum, and bismuth; and    an inorganic oxide precursor BMOx based on at least one inorganic element B selected from the group consisting of scandium, yttrium, lanthanum, and the lanthanoids;wherein AMOx and BMOx are capable of forming a mixed inorganic oxide.
With the coating composition of the invention an inorganic oxide coating can be made on a substrate, which coating unexpectedly shows improved hydrolytic stability compared to a coating not comprising element B or component BMOx. The coating is thus able to retain its functional properties over an extended time while being subjected to variations in temperature and humidity. A further advantage of the coating composition of the invention, especially such liquid coating composition comprising solvent, is the (storage) stability of the coating liquid over time.